Method and apparatus for determining endpoint of semiconductor element fabricating process

ABSTRACT

Standard patterns of differential values of interference light that correspond to a predetermined step height of the first material being processed and standard patterns of differential values of interference light that correspond to a predetermined remaining mask layer thickness of the material are set. These standard patterns use wavelengths as parameters. Then, the intensities of interference light of multiple wavelengths are measured for a second material that has the same structure as the first material. Actual patterns with wavelength as parameter are determined from differential values of the measured interference light intensities. Based on the standard patterns and the actual patterns of the differential values, the step height and the remaining mask layer thickness of the second material are determined.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 10/978,412, filedNov. 2, 2004, now U.S. Pat. No. 7,009,715 which is a continuation ofU.S. application Ser. No. 09/946,504, filed Sep. 6, 2001 (now U.S. Pat.No. 6,903,826), and is related to U.S. application Ser. No. 09/793,624,filed Feb. 27, 2001 (now abandoned). The entirety of the contents andsubject matter of all of the above is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method and apparatus which processesa material by using a method and apparatus for determining an endpointof a semiconductor device manufacturing process. More specifically, thepresent invention relates to a method and apparatus for determining anendpoint of semiconductor device manufacturing process and a method andapparatus for processing a material by using the endpoint determiningmethod and apparatus, which detects an etch quantity of, or depositionon, a material being processed in a semiconductor integrated circuitmanufacturing process. More specifically, the present invention relatesto a method and apparatus for measuring an etch depth and a thickness ofa material being processed and a method and apparatus for processing amaterial by using the etch depth/thickness measuring method andapparatus, which can precisely measure etch quantity of each of variouslayers formed over a substrate during an etching process using plasmadischarge and thereby realize a desired etch depth and thickness of eachlayer.

In the manufacture of semiconductor wafers, dry etching has been in wideuse in etching layers of various materials, particularly dielectrics,and in forming patterns over the wafer surface. What is most importantin controlling process parameters is to accurately determine an etchingendpoint during the processing of these layers at which desired etcheddepth and film thickness are reached and the etching is stopped.

During the dry etching of a semiconductor wafer, the intensity of lightof a specific wavelength in a plasma beam changes as the etching of aparticular film proceeds. An example method currently available fordetermining an endpoint of the semiconductor wafer etching processinvolves detecting a change in the intensity of light of a particularwavelength emitted from a plasma during the dry etching and, based onthe result of detection, determining an etching process endpoint for aparticular film. In this case, it is necessary to prevent an erroneousdetection caused by noise-induced variations of a detected waveform.Methods for detecting light intensity variations with high precisionincludes those described in Japanese Patent Unexamined Publication Nos.61-53728 and 63-200533. Noise reduction is achieved by a moving averagemethod described in Japanese Patent Unexamined Publication No. 61-53728and by approximation processing using a first-order least squares methoddescribed in Japanese Patent Unexamined Publication No. 63-200533.

As semiconductors are fabricated in increasingly microfine structuresand in higher circuit densities in recent years, an open area ratio (anarea of a semiconductor wafer to be etched with respect to its overallarea) decreases, weakening the intensity of light of a specificwavelength from a reactive product that is taken into an photodetectorfrom an optical sensor. As a result, a level of signal sampled from thephotodetector becomes small, making it difficult for an endpointdetermining unit to determine the etching process endpoint reliablybased on the sampled signal from the photodetector.

In stopping the etching process after detecting the etching processendpoint, it is important that the remaining thickness of a dielectriclayer be equal to a predetermined value. The conventional techniquesmonitor an overall process by using a time-thickness control methodwhich assumes that the etch rate in each layer is constant. The etchrate is determined in advance as by processing a sample wafer. Thisapproach uses a time monitor technique and stops the etching processwhen a length of time corresponding to a predetermined film thickness tobe etched has elapsed.

An actual layer, for example, an SiO₂ layer formed by the LPCVD (lowpressure chemical vapor deposition) technique, however, is known to havea low reproducibility in terms of thickness. An allowable error ofthickness due to LPCVD process variations is equal to about 10% of aninitial thickness of the SiO₂ layer. Hence, the time monitor techniquecannot precisely measure the actual final thickness of the SiO₂ layerremaining on a silicon substrate. The actual thickness of the remaininglayer is measured at a final step by a technique using a standardspectroscopic interferometer. When it is decided that the wafer isoveretched, the wafer is discarded as a faulty product.

It is also known that the insulating film etching apparatus hasperformance variations with elapse of time, such as the etch ratefalling as the etching operation is repeated. In some cases the etchingmay inadvertently stop while in process. These problems need to besolved. In addition, it is important to monitor etch rate variationsover time in assuring a stable execution of the process. Theconventional method, however, monitors only the time in determining theetching process endpoint and does not provide an appropriate means ofdealing with variations over time of the etch rate. When the duration ofetching is as short as about 10 seconds, the endpoint determining methodmust reduce a decision preparation time and the decision interval bemade short enough. These requirements are not met satisfactorily.Another problem is that an etched area of the insulating film is in manycases less than 1%, which means that a change in intensity ofplasma-induced light emitted from a reactive product created duringetching is small. Hence an endpoint determining system capable ofdetecting even very small changes is required, but no practical andinexpensive systems with such a capability are available.

Among other etching process endpoint determining methods forsemiconductor wafers are those using an interferometer which aredisclosed in Japanese Patent Unexamined Publication Nos. 5-179467,8-274082, 2000-97648 and 2000-106356. These methods using aninterferometer apply a monochromatic radiation from a laser at anorthogonal incidence angle to a wafer that includes a stacking structureof different materials. In a structure in which an SiO₂ layer is stackedover an Si₃N₄ layer, for example, the radiated light reflected by theupper surface of the SiO₂ layer and the radiated light reflected by aboundary surface formed between the SiO₂ layer and the Si₃N₄ layercombine to form interference fringes. The reflected light is radiatedonto an appropriate detector which generates a signal whose magnitudechanges according to the thickness of the SiO₂ layer being etched.During the etching process, as soon as the upper surface of the SiO₂layer is exposed, it becomes possible to monitor continuously andprecisely the etch rate and the present thickness of the layer beingetched. Another method is also known which uses a plasma, rather than alaser, to emit predetermined light which is measured by a spectrometer.

SUMMARY OF THE INVENTION

The contents of the literatures cited above may be summarized asfollows.

In Japanese Patent Unexamined Publication No. 5-179467, three colorfilters, red, green and blue, are used to detect interference light(plasma light) to determine an endpoint of the etching process.

In Japanese Patent Unexamined Publication No. 8-274082 (U.S. Pat. No.5,658,418), a change over time of an interference waveform made up oftwo wavelengths and its differential waveform are used to count extremalvalues of interference waveform (maximum and minimum of waveform:zero-cross point of differential waveform). By measuring the time ittakes for the count value to reach a predetermined value, an etch rateis calculated. Based on the calculated etch rate, a remaining etch timerequired to reach a predetermined film thickness is determined and theetching process is stopped accordingly.

Japanese Patent Unexamined Publication No. 2000-97648 determines awaveform of difference (with wavelength as parameter) between aninterference light intensity pattern (with wavelength as parameter)before the process and an interference light intensity pattern (withwavelength as parameter) after or while in the process; and compares thedifference waveform with difference waveforms stored in database todetermine a step height (film thickness).

Japanese Patent Unexamined Publication No. 2000-106356 relates to arotary film application apparatus and measures a change over time ofinterference light of multiple waveforms to determine a film thickness.

U.S. Pat. No. 6,081,334 measures characteristic behaviors in a changeover time of interference light and stores them in database, andcompares the measured interference waveform with the database todetermine the endpoint of the etching process. With the endpointdetermined, an operator is then prompted to change an etching processcondition.

These known methods have the following problems.

(1) In the etching process using a mask layer (e.g., resist, nitridefilm and oxide film), interference light from the mask layer issuperimposed on interference light from the object material beingetched.

(2) In the etching of the material to be processed (e.g., siliconsubstrate and mask layer formed over the silicon substrate), because themask layer as well as silicon substrate is etched, simply measuring theetch amount of the material (etch depth) may not be able to correctlymeasure the etch amount of the silicon substrate.

(3) Wafers for a mass production process have distribution variations,due to device structures, in the initial thickness of the mask layer andin the initial thickness of the material to be etched so thatinterference light from differing film thicknesses may be superimposedon the target interference light.

For the reasons cited above, it is difficult to correctly measure andcontrol with a required precision the etch depth and remaining thicknessof a film being processed (layer to be processed in the semiconductormanufacture), particularly in the plasma etching.

It is an object of the present invention to provide a method andapparatus for determining an endpoint of semiconductor devicemanufacturing process and a method and apparatus for processing amaterial by using the endpoint determining method and apparatus, whichcan solve the above-described problems experienced with the conventionaltechniques.

Another object of the present invention is to provide a method andapparatus for determining an endpoint of semiconductor devicemanufacturing process and a method and apparatus for processing amaterial by using the endpoint determining method and apparatus, whichcan precisely measure online the actual etch depth and remainingthickness of a layer being processed in plasma processing, particularlyin plasma etching, and thereby determine an endpoint of the process.

Still another object of the present invention is to provide an etchingprocess which can control online the etch depth and thickness of eachlayer in semiconductor devices to desired values with high precision.

A further object of the present invention is to provide an apparatusthat can precisely measure online the actual etch depth and thickness ofa layer being processed.

To solve the problems experienced with the conventional techniques andachieve the above objectives, the inventors of this invention propose amethod that determines time-differential waveforms of interference lightproduced by interference between a plurality of wavelengths and, basedon the time-differential waveforms, determines patterns representingwavelength dependencies of the interference waveform differential values(i.e., patterns of differential values of interference waveforms usingwavelength as parameter), and measures the film thickness by using thesepatterns.

The reasons for using the patterns representing the wavelengthdependencies of the time-differential values of interference waveformsin this invention are as follows.

Because the measurement assumes the in-situ (realtime) measuring duringthe etching, the thickness of a layer being processed are constantlychanging. Hence, the interference waveforms can be time-differentiated.Further, this differentiation can remove noise from the interferencewaveforms.

Further, the refractive indices of the materials to be etched (e.g.,silicon substrate and mask layer nitride film) are different withrespect to the wavelength. Therefore, by measuring the interferencelight of multiple wavelengths, it is possible to detect characteristicchanges (dependent on film thickness) of individual substances.

According to one aspect, the present invention provides a method ofmeasuring an etch depth and a thickness of a material being processed,comprising:

a) a step of setting standard patterns of differential values ofinterference light from a first material to be processed that correspondto a predetermined etch quantity of the first material including a masklayer, the standard patterns using the wavelength as a parameter;

b) a step of setting standard patterns of differential values ofinterference light from the mask layer of the first material thatcorrespond to a predetermined etch quantity of the mask layer, thestandard patterns using the wavelength as a parameter;

c) a step of measuring intensities of interference light of multiplewavelengths from a second material being processed of the same structureas the first material and determining actual patterns of differentialvalues of the measured interference light intensities, the actualpatterns using the wavelength as a parameter; and

d) a step of determining an etch quantity of the second material basedon the standard patterns of the first material, the standard patterns ofthe mask layer, and the actual patterns.

This invention therefore can provide a method of measuring an etch depthand a thickness of a material being processed and a method of processinga sample of the material by using the etch depth/thickness measuringmethod, which can precisely measure online the actual etch quantity inplasma processing, especially plasma etching.

Further, it is also possible to provide an etching process that cancontrol online the etch quantity of each layer of semiconductor devicesto a desired value with high precision. It is also possible to providean etch depth and thickness measuring apparatus that can preciselymeasure online the actual etch quantity of a layer being processed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an overall configuration of asemiconductor wafer etching apparatus having an etch quantity measuringapparatus according to a first embodiment of the present invention.

FIG. 2A is a vertical cross section of a wafer being etched.

FIGS. 2B and 2C are diagrams showing examples of actual waveforms ofinterference light with different wavelengths.

FIGS. 3A and 3B are diagrams showing differential coefficient timeseries data of interference light corresponding to a step height(distance from the mask surface a, b, c to the etched surface of siliconwafer A, B, C) and remaining film thickness of the mask surface a, b, cin FIGS. 2A, 2B, with wavelength taken as parameter.

FIG. 4 is a flow chart showing a procedure for determining the stepheight in the wafer being processed and the remaining film thicknessduring the etching process performed by the etch quantity measuringapparatus.

FIG. 5 is a block diagram showing an overall configuration of asemiconductor wafer etching apparatus having an etch depth measuringapparatus according to a variation of the first embodiment of thepresent invention.

FIG. 6 is a flow chart showing an operation of the embodiment of FIG. 5.

FIG. 7 is a diagram showing measurements of an etch depth in theembodiment of FIG. 5.

FIG. 8 is a block diagram showing an overall configuration of asemiconductor wafer etching apparatus having a remaining film thicknessmeasuring apparatus according to a second embodiment of the presentinvention.

FIG. 9 is a vertical cross section of a wafer being etched.

FIG. 10 is a flow chart showing an operation of the embodiment of FIG.8.

FIG. 11 is a diagram showing measurements of a remaining polysiliconthickness and a regression line in the embodiment of FIG. 8.

FIG. 12 is a block diagram showing an overall configuration of asemiconductor wafer etching apparatus having an etch depth measuringapparatus according to a third embodiment of the present invention.

FIG. 13 is a vertical cross section of a wafer being etched.

FIG. 14 is a flow chart showing an operation of the embodiment of FIG.12.

FIG. 15 is a block diagram showing an overall configuration of asemiconductor wafer etching apparatus having a remaining film thicknessmeasuring apparatus according to a fourth embodiment of the presentinvention.

FIG. 16 is a vertical cross section of a wafer being etched.

FIG. 17 is a flow chart showing an operation of the embodiment of FIG.15.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described. In the followingembodiments, constitutional members with similar functions to those ofthe first embodiment carry like reference numbers and their detaileddescriptions are omitted. As the endpoint determining method used in thesemiconductor device manufacturing process according to the presentinvention, we will describe a method of measuring an etch quantity(etched depth and film thickness) in a wafer etching process. It shouldbe noted that the present invention is not limited to this method andcan also be applied to a method of measuring an amount of deposition(deposited film thickness) or the like in such film deposition processesas plasma CVD and sputtering.

Now, a first embodiment of the present invention will be described byreferring to FIG. 1 to FIG. 4.

In this embodiment, when plasma etching is to be performed on a materialto be processed such as a semiconductor wafer, standard patterns P_(S)and P_(M) are set which represent wavelength dependencies ofdifferential values of interference light intensities as related to theetch quantities of a sample material (sample wafer) and of a mask layeron the wafer (with wavelength taken as parameter). Next, for an actualwafer with the same structure as the sample wafer, measurements aretaken of intensities of interference light at a plurality of wavelengthsused in the actual processing to determine actual patterns representingwavelength dependencies of differential values of the measuredinterference light intensities (with wavelength taken as parameter). Thestandard patterns and the actual patterns of the light intensitydifferential values are compared to determine an etch quantity (endpointof the process) for the actual wafer.

First, with reference to FIG. 1, an overall configuration of thesemiconductor wafer etching apparatus having an etch quantity (etcheddepth in actual wafer and film thickness) measuring apparatus accordingto the present invention will be explained. An etching apparatus 1 has avacuum chamber 2 into which is introduced an etching gas which isdecomposed by a microwave power to create a plasma 3 which in turnetches a material 4 such as a semiconductor wafer placed on a sampletable 5. A measuring light source (e.g., halogen light source) of aspectroscope 11 of an etch quantity (e.g., etched depth and filmthickness) measuring apparatus 10 radiates light of multiplewavelengths, which is guided through an optical fiber 8 into the vacuumchamber 2 and applied to the wafer 4 at an orthogonal incidence angle.The wafer 4, as shown in FIG. 2A, has a silicon 40 as a material to beetched and a nitride film 41 as a mask layer. The radiated light isreflected by the mask layer as indicated by 9A and by the siliconsurface clear of the mask layer as indicated by 9B, and these reflectedrays of light form interference light. That is, the interference lightis an interference component generated by the step height between themask layer and the silicon. The reflected light 9A from the mask layercomprises light 9 a reflected by the surface of the nitride film andlight 9 b reflected by a boundary surface between the nitride film andthe silicon. These reflected light also forms interference light. Thatis, this interference light is an interference component produced by areduction in the mask layer thickness. The interference light isintroduced through the optical fiber 8 into the spectroscope 11 of theetch quantity measuring apparatus 10 which, based on the receivedinterference light, measures the etched silicon depth and the mask layerthickness and determines an endpoint of the process (i.e., end ofetching).

The etch quantity measuring apparatus 10 has a spectroscope 11, firstdigital filter circuits 12, 22, differentiators 13, 23, second digitalfilter circuits 14, 24, differential waveform pattern databases 16, 26,differential pattern comparators 15, 25, an endpoint determining circuit30 for determining an endpoint of the etching process according to theresults of these comparators, and a display 17 for displaying a resultof decision made by the endpoint determining circuit 30.

FIG. 1 shows a functional configuration of the etch quantity measuringapparatus 10, and the actual configuration of the etch quantitymeasuring apparatus 10 excluding the display 17 and the spectroscope 11may be comprised of a CPU, storage devices such as ROM for storing anetched depth and film thickness measuring program and various dataincluding interference light differential waveform pattern database, RAMfor storing measured data and external storage devices, a datainput/output device, and a communication control device. This can alsobe applied to other embodiments, such as one shown in FIG. 1.

The intensities of light of multiple wavelengths from the wafer that arereceived by the spectroscope 11 are detected as current signalscorresponding to the light intensities and transformed into voltagesignals. Signals representing a plurality of particular wavelengthsoutput as sampling signals from the spectroscope 11 are stored as timeseries data yij, y′ij in a storage device such as RAM not shown. Firstand second wavelength bands of the time series data yij, y′ij aresmoothed by the first digital filter circuits 12, 22 and stored assmoothed time series data Yij, Y′ij in a storage device such as RAM.Based on the smoothed time series data Yij, Y′ij, the differentiators13, 23 calculate time series data dij, d′ij of differential coefficientvalues (first differential value or second differential value) which arestored in a storage device such as RAM. The time series data dij, d′ijof differential coefficient values are smoothed by the second digitalfilter circuits 14, 24 and stored as smoothed differential coefficienttime series data Dij, D′ij in a storage device such as RAM. From thesmoothed differential coefficient time series data Dij, D′ij aredetermined actual patterns representing the waveform dependencies ofdifferential values of the interference light intensities (with thewavelength taken as parameter).

To obtain different actual patterns for the first and second wavelengthbands, the apparatus of FIG. 1 is configured as follows. When the firstand second wavelength bands are equal, the differential coefficients ofthe first digital filter circuits 12, 22 are set to different values. Inthat case, the values of the differentiators 13, 23 may be set to equalor different values and the differential coefficients of the seconddigital filter circuits 14, 24 set to equal or different values. Whenthe first and second wavelength bands are different, the differentialcoefficients of the first digital filter circuits 12, 22 are set toequal or different values and the values of the differentiators 13, 23set to equal or different values. The differential coefficients of thesecond digital filter circuits 14, 24 may also be set to equal ordifferent values.

In the configuration of FIG. 1, the circuit comprising the first digitalfilter circuit, the differentiator, the second digital filter circuit,the differential waveform pattern database and the differential patterncomparator is provided for each of the first and second waveform bands.Instead of this configuration, one such circuit may be provided commonlyfor both the first and second wavelength bands, and the differentialcoefficients and others may be switched at predetermined time intervalsto produce the actual pattern alternately for the first wavelength bandand for the second wavelength band.

The differential waveform pattern database 16 is preset with adifferential waveform pattern data value P_(S)j of interference lightintensity for the first wavelength band that corresponds to the stepheight between the silicon and the mask layer, the silicon being anobject to be checked for the etch quantity. The differential waveformpattern database 26 is preset with a differential waveform pattern datavalue P_(M)j of interference light intensity for the second wavelengthband that corresponds to the thickness of the mask layer on the siliconwafer. The differential pattern comparator 15 compares the actualpattern corresponding to the step height with the differential waveformpattern data value P_(S)j to determine the step height (depth from thesurface of the mask layer to the bottom of a groove etched in thesilicon wafer). The differential pattern comparator 25 compares theactual pattern corresponding to the mask layer thickness with thedifferential waveform pattern data value P_(M)j to determine thethickness of the mask layer (remaining film thickness). As a result, theetch quantity of the material being etched, i.e., etched depth, isdetermined and displayed on the display 17.

Although this and the following embodiments show configurations withonly one spectroscope 11, a plurality of spectroscopes 11 may beprovided when it is desired to measure and control a wider area of thewafer.

FIG. 2A illustrates a vertical cross section of the silicon wafer 4while in the etching process, and FIG. 2B and FIG. 2C show actualpatterns of interference light waveform. In FIG. 2A the mask layer 41 isformed over the silicon substrate 40. In this etching process, thesilicon substrate is a material to be etched and this type of processingis called an STI (Shallow Trench Isolation) etching to isolate devices.

The light of multiple wavelengths emitted from the spectroscope 11 isradiated at an orthogonal incident angle onto the wafer 4, whichincludes a stacked structure of a material to be etched and a masklayer. The light 9 applied to the mask layer 41 is reflected by theupper surface of the mask layer 41 as indicated by 9 a and alsoreflected by a boundary surface between the mask layer 41 and thesilicon substrate 40 as indicated by 9 b. These reflected rays of lightcombine to form interference light. The light 9 introduced to an areathat is clear of the mask layer 41 and etched is reflected by the uppersurface of the silicon substrate 40 as indicated by 9B. This reflectedlight 9B interferes with light 9A (comprised of 9 a and 9 b) reflectedby the mask layer 41 to form interference light. The positions at whichthe rays of light 9B and 9 a are reflected change to A(a), B(b) and C(c)as the etching process proceeds. The reflected rays of light areintroduced into the spectroscope 11 which generates a signal whosemagnitude changes according to the thicknesses of the silicon substrate40 being etched and the mask layer.

As shown in FIG. 2B, the waveform of interference light in the longwavelength range (second wavelength band: 700 nm for example) slowlychanges as the etching process proceeds. The waveform of interferencelight in the short wavelength range (first wavelength band: 300 nm forexample) has a long-cycle wave and a short-cycle wave superimposed andchanges accordingly, as shown in FIG. 2C. This is because theinterference light in the long wavelength range represents a change inthe interference component caused by a reduction in the mask layerthickness (a, b, c surfaces of FIG. 2A) while the interference light inthe short wavelength range represents a change in the interferencecomponent caused by the step height 44 between the silicon substrate tobe etched and the mask layer (depth difference between the mask layersurface a, b, c and etched silicon substrate surface A, B, C). Based onthe smoothed time series data Yij, Y′ij of the interference light ofthese multiple wavelengths, calculations are performed to producedifferential coefficient time series data dij, j′ij of firstdifferential value or second differential value. FIG. 2B shows the firstdifferential value and second differential value of the interferencelight with a wavelength of 700 nm and FIG. 2C shows the firstdifferential value and second differential value of the interferencelight with a wavelength of 300 nm.

As can be seen from FIG. 2B and FIG. 2C, this differentiation processingclearly distinguishes the change in the interference component due to areduction in the mask layer thickness from the change in theinterference component due to the step height between the siliconsubstrate and the mask layer. This is because the refractive index ofthe material being etched (e.g., refractive indices of silicon andnitride film or mask layer and refractive index of vacuum space in thegrooved portion) differs depending on the wavelength. The presentinvention focuses on this fact and is characterized in that itdetermines the step height 44 between the silicon substrate and the masklayer from the interference light in the short wavelength range andaccurately measures the reduction in the mask layer thickness (remainingthickness of the mask layer 41) from the interference light in the longwavelength range.

FIG. 3A shows pattern data of first differential waveform of theinterference light and FIG. 3B shows pattern data of second differentialwaveform of the interference light.

PA, PB and PC in FIG. 3A and FIG. 3B represent differential waveformpattern data for etch quantities A (step height 44=300 nm), B (stepheight=400 nm), C (step height=500 nm) of FIG. 2A. Similarly, Pa, Pb andPc in FIG. 3A and FIG. 3B represent differential waveform pattern datafor etch quantities a (remaining mask layer thickness 42=95 nm), b(remaining mask layer thickness=65 nm), c (remaining mask layerthickness=35 nm) of FIG. 2A. The etched depth 43 in the siliconsubstrate is 205 nm for the position A of FIG. 2A, 335 nm for position Band 465 nm for position C.

As can be seen from FIG. 3A and FIG. 3B, the first differential waveformpattern and second differential waveform pattern of the interferencelight are characteristic of each etch quantity of the material beingetched. These patterns change from one material to another, soexperiments are conducted in advance to obtain data for a variety ofmaterials and a range of etch quantity used in the process and to storein storage devices (16, 26) first differential waveform pattern andsecond differential waveform pattern as the standard patterns.

Next, by referring to the flow chart of FIG. 4, we will explain aprocedure for determining the etch quantity of the material being etchedby the etch quantity measuring apparatus 10.

First, a target etch quantity (i.e., a target step height and a targetremaining mask layer thickness) is set, and the differential patternsP_(S)j, P_(M)j corresponding to the target step height and the targetremaining mask layer thickness and decision criteria σ_(S0), σ_(M0) arealso set (step 400 and 420). That is, from the differential valuestandard patterns PA, PB, PC for a plurality of wavelengths as shown inFIG. 3A and FIG. 3B that are stored in advance in the differentialwaveform pattern database 16, one differential pattern corresponding tothe target step height is set. Similarly, from the differential valuestandard patterns Pa, Pb, Pc for a plurality of wavelengths as shown inFIG. 3A and FIG. 3B that are stored in advance in the differentialwaveform pattern database 26, one differential pattern corresponding tothe target remaining mask layer thickness is set.

In the next step, sampling of the interference light (every 0.25–0.4second) is started (step 402). That is, when the etching process starts,a sampling start command is issued. The intensities of emitted light ofmultiple wavelengths that change during the etching process are detectedby the photodetector of the spectroscope 11 as the light detectionsignals of voltage representing the light intensities. The lightdetection signals of the spectroscope 11 are converted into digitalsignals which are the sampling signals yi,j, y′i,j.

Next, the multi-wavelength output signals yi,j, y′i,j from thespectroscope 11 are filtered by the first digital filters 12, 22 tocalculate time series data Yi,j, Y′i,j (steps 404, 424).

Next, the differentiators 13, 23 calculate the differential coefficientsdi,j, d′i,j by the S-G method (Savitzky-Golay method) (steps 406, 426).That is, the differentiation processing (S-G method) determines acoefficient (first or second order) di of the signal waveform. Further,the second digital filters 14, 24 smooth the coefficient to obtaindifferential coefficient time series data Di,j, D′i,j (steps 408, 428).Then, the differential pattern comparator 15 calculates a value ofσ=Σ(Di,j−P_(S)j)² for the step height (step 410). Likewise, thedifferential pattern comparator 25 calculates a value ofσ′=Σ(D′i,j−P_(M)j)² for the reduction in the mask layer thickness (orremaining mask layer thickness) (step 430). Further, the endpointdetermining circuit 30 checks the relations of σ≦σ_(S0) and σ′≦σ_(M0)(step 412). When the relations of σ≦σ_(S0) and σ′≦σ_(M0) hold, it isdecided that the step height and the remaining mask layer thicknessreach their respective predetermined values and the etching processtermination step is initiated. At the same time, the result of this stepis displayed on the display 17. When any of the relations σ≦σ_(S0) andσ′≦σ_(M0) fails to be met, the procedure returns to steps 404, 424.Finally, the sampling termination is set (step 414).

Here, let us explain about the calculation of the smoothed differentialcoefficient time series data Di, D′i. The digital filter circuits 12,22, 14, 24 may use secondary Bataworth lowpass filters, for example. Thedigital filter circuits 12, 22 have the same configuration and theircoefficients b, a may or may not be equal. Here, only the digital filtercircuit 12 will be described. The secondary Bataworth lowpass filtercalculates the smoothed time series data Yi from the equation (1).Yi=b1yi+b2yi−1+b3yi−2−[a2Yi−1+a3Yi−2]  (1)

Here, coefficients b, a vary depending on the sampling frequency andcutoff frequency. The coefficient values of the digital filters maydiffer between the wavelength range for the step height (firstwavelength band, e.g., 275 nm to 500 nm) and the wavelength range forthe mask layer thickness reduction (remaining mask layer thickness)(second wavelength band, e.g., 525 nm to 750 nm). For the step heightwavelength range, a2=−1.143, a3=0.4128, b1=0.067455, b2=0.13491,b3=0.067455 (sampling frequency of 10 Hz, cutoff frequency of 1 Hz). Forthe remaining mask layer thickness wavelength range, a2=−0.00073612,a3=0.17157, b1=0.29271, b2=0.58542 and b3=0.29271 (cutoff frequency of0.25 Hz).

The second differential coefficient time series data di, d′i arecalculated as follows by the differentiators 13, 23 according to thepolynomial adaptation smoothing differential method (S-G method) thatuses 5-point time series data Yi from equation (2).j=2di=d′i=ΣwjYi+j  (2)j=−2

As for the weight w, w−2=2, w−1=−1, w0=−2, w1=−1, and w2=2.

The differentiators 13, 23 may or may not have equal values of j.

The smoothed differential coefficient time series data Di, D′i can bedetermined by the second digital filter circuits 14, 24 (secondaryBataworth lowpass filters) from equations (3) and (4) using thedifferential coefficient time series data di, d′i. The coefficients a, bmay be different between the second digital filter circuits 14, 24.Di=b1di+b2di−1+b3di−2−[a2Di−1+a3Di−2]  (3)D′i=b1d′i+b2d′i−1+b3d′i−2−[a2D′i−1+a3D′i−2]  (4)

In this way, the etch quantity measuring apparatus 10 of FIG. 1 candetermine the step height and the remaining mask layer thickness bysetting at least one standard pattern of differential value for multiplewavelengths, such as PA, PB, PC, Pa, Pb and Pc shown in FIG. 3A and FIG.3B, measuring the intensities of interference light of multiplewavelengths from the silicon wafer being processed, determining anactual pattern of differential value of measured interference lightintensity of each wavelength, and comparing the standard patterns andthe actual patterns of differential value. When it is desired to detectthe silicon etched depth of 335 nm, i.e., the position B of FIG. 2, theprocedure involves setting in advance the differential value standardpatterns PB, Pb corresponding to the etch quantity (step height andremaining mask layer thickness) B, b for multiple wavelengths, checkingwhether matching factors of the actual patterns with respect to thestandard patterns for multiple wavelengths come within the decisioncriteria σ_(S0), σ_(M0), and, if so, deciding that the step height 44 isnow 400 nm and the remaining mask layer thickness 42 is 65 nm (etcheddepth 43 in silicon substrate is 335 nm). The standard pattern may usethe first differential pattern or the second differential pattern, orboth. With this embodiment, by determining the etch quantity of thematerial being etched (step height and remaining mask layer thickness),it is possible to accurately detect when the silicon etched depth is 335nm, for example.

Next, a variation of the first embodiment intended to improve theprecision of measuring the etch quantity when the interference lightintensity being measured contains noise components, will be explained byreferring to the block diagram of FIG. 5 showing the configuration ofthis variation and the flow chart of FIG. 6. This variation may beapplied where the pattern differs from one wafer to another and thusdifferent wafers have different etching conditions (such as dischargeconditions) and therefore different interference waves. First, a targetprocess depth (here, the target etch depth: 43 in FIG. 2A) in thematerial to be processed (silicon wafer and mask layer) is set (step550). Next, the step height, the differential waveform pattern for theremaining mask layer thickness and the convergence decision criteria(step height: P_(S)j, σ_(S0), remaining mask layer thickness: P_(M)j,σ_(M0)) are read out from the differential waveform pattern databases16, 26 and set in the differential pattern comparators 15, 25. When theetching process is started, the sampling of the interference light isstarted (step 502). Next, as in the steps 404, 410, 424–430 of FIG. 4,steps 504–510, 524–530 are executed. The light from the spectroscope 11in the short wavelength range and the long wavelength range are suppliedto the first digital filter circuits 12, 22 and differentiators 13, 23and to the second digital filter circuits 14, 24 to determine smootheddifferential coefficient time series data Di,j, D′i,j. The smootheddifferential coefficient time series data Di,j, D′i,j are compared withthe differential patterns P_(S)j, P_(M)j set beforehand in thedifferential pattern comparators 15, 25 to calculate a step height Siand a mask layer thickness reduction (remaining mask layer thickness) Miat that time (steps 515, 535). When σ>σ_(S0) (or σ>σ_(M0)), Si (or Mi)that was obtained at time of step 515 (or step 535) is not stored andthe processing by the recurrent analyzer 19 excludes the data ofremaining mask layer thickness at this point in time.

The step height and the remaining mask layer thickness obtained at steps515, 535 are stored in the data recorders 18, 28 as time series data Si,Mi. Using the stored old time series data Sj, Mj, the recurrentanalyzers 19, 29 calculate a first-order regression line Y=Xa*t+Xb (Y:etch quantity (step height and remaining mask layer thickness), t: etchtime, |Xa|: etch rate; the absolute value of Xa represents an etch rate,Xb: initial mask layer thickness). From this regression line, an etchquantity (step height: S, remaining mask layer thickness: M) at presenttime is calculated (steps 516, 536). Here, the etch time, the etch rate,the initial mask layer thickness, the remaining mask layer thickness,etc. are process quantities (in this case, etch quantities).

Next, the endpoint determining circuit 30 calculates the process depth(S−M, or 43 in FIG. 2A) from these etch quantities S, M. This value iscompared with the target process depth. If it is found to be equal to orlarger than the target process depth, it is decided that the etchquantity of the material being processed has reached the predeterminedvalue. The etching process is then terminated and the result isdisplayed on the display 17. When the calculated process depth issmaller than the target process depth, the procedure returns to steps504, 524. As a final step, the sampling termination is set (step 514).

FIG. 7 shows a result of measurement of the silicon depth obtained inthe above embodiment (values calculated by steps 516, 536). The diagramshows changes over time of the silicon process depth and of the depthfrom the mask layer, clearly showing how the etching process wasperformed during the STI process. In this embodiment, the etching wasdone until the silicon depth was 506 nm (step height: 529 nm).

With the etch quantity measuring apparatus of this embodiment describedabove, the etch quantity of the material being processed in thesemiconductor device manufacturing process can be measured accurately.The use of this system can therefore provide a method of etching amaterial with high precision. Further, unlike the first embodiment, thisvariation of the first embodiment can measure an arbitrary etch quantityof the material other than those set by the standard patterns.

In FIG. 5, the configuration comprising the first digital filtercircuit, differentiator, second digital filter circuit, differentialwaveform pattern database, differential pattern comparator and recurrentanalyzer is provided for each of the first and second wavelength bands.It is also possible to provide only one such configuration commonly forboth the first and second wavelength bands and to switch between thedifferential coefficients at predetermined intervals to produce anactual pattern alternately for the first and second wavelength bands.

Next, a second embodiment of the present invention will be described byreferring to FIGS. 8, 9, 10 and 11. The configuration of this embodimentshown in FIG. 8 is identical with the configuration 11–19 of FIG. 5 forone wavelength band and the operation of the endpoint determiningcircuit 130 is different from that of the endpoint determining circuit30 of FIG. 5. The structure of the material to be etched is shown inFIG. 9. The area of a polysilicon 50 processed by this etching processlies where there is no mask layer 51 (e.g., nitride film andphotoresist). The observed interference light is produced by theinterference between light 90A reflected by the surface of thepolysilicon 50 and light 90B reflected by an underlying oxide film 52. Amethod of measuring the etch quantity (remaining thickness 53: thicknessof polysilicon measured from the underlying oxide film) by measuring theinterference light will be explained with reference to the flow chart ofFIG. 10.

First, a target remaining thickness of the material to be processed(polysilicon), and all standard differential patterns (P_(Z)j)associated with the polysilicon film thickness and convergence decisioncriteria σ_(Z0) stored in advance in the differential waveform patterndatabase 16 are set in the differential pattern comparator 15 (step600). When the etching process is started, the interference lightsampling is started (step 602). The light of multiple wavelengths fromthe spectroscope 11 is supplied to the first digital filter 12,differentiator 13 and second digital filter 14 to determine smootheddifferential coefficient time series data Di,j in a way similar to thesteps 404–410 in the first embodiment. The smoothed differentialcoefficient time series data Di,j are compared with the differentialpattern P_(Z)j preset in the differential pattern comparator 15 tocalculate the remaining film thickness Zi at that point in time (step615). When σ>σ_(Z0), the Zi value obtained at this point in time by step615 is not saved and the processing by the recurrent analyzer 19excludes the data of the remaining film thickness at this point in time.

The remaining film thickness obtained at step 615 is stored as timeseries data Zi in data recorder 18. By using the stored old time seriesdata Zi, the recurrent analyzer 19 determines the first-order regressionline Y=Ya*t+Xb (Y: remaining film thickness, t: etch time, |Xa|: etchrate, Xb: initial film thickness) and, based on the regression line,calculates the remaining film thickness Z at the current time (step616).

Next, the endpoint determining circuit 130 compares the remaining filmthickness Z and the target remaining film thickness. If the remainingfilm thickness Z is found to be equal to or less than the targetthickness, it is decided that the etch quantity of the material hasreached the predetermined value and the result is displayed on thedisplay 17. When the remaining film thickness Z is larger than thetarget value, the procedure returns to step 604. As a final step, thesampling termination is set (step 614).

When at step 618 it is decided that the target remaining film thicknessis smaller than the differential patterns P_(Z)j of the polysilicon filmthickness stored in advance in the differential waveform patterndatabase 16, the following processing is performed before ending theetching process. When the remaining film thickness Z is equal to theminimum film thickness Ym available in the differential waveform patterndatabase 16 of the polysilicon film thickness, an etching time at whichthe remaining film thickness will become the target remaining filmthickness Y_(T) (t_(T)=[Y_(T)−Xb]/Xa) is calculated from the first-orderregression line Y=Xa*t+Xb and the etching process is continued up to thetime t_(T).

FIG. 11 shows a result of measurement of the polysilicon's remainingthickness in this embodiment. This diagram represents a case where byusing the differential waveform pattern database that has polysiliconfilm thickness data down to a minimum thickness Ym of 45 nm, a targetremaining film thickness Yt of 20 nm is estimated. The diagram clearlyshows from the first-order regression line that the etching time atwhich the target remaining film thickness of 20 nm will be reached is 96seconds. It is therefore possible to make a decision on a remaining filmthickness for which the associated differential waveform patterndatabase is not available.

The absolute value of the first-order regression line (=|Xa|) representsan etch rate. By controlling the etch rates in the mass production, thestatus of the etching apparatus can be controlled. That is, when theetch rate is within an allowable range, it can be decided that theetching apparatus is operating in normal condition. If the etch rate isoutside the allowable range, the etching apparatus can be determined asabnormal.

Further, an intersecting point of the first-order regression line (=Xb)represents an initial thickness of the material. Controlling thisinitial thickness in the mass production can control the depositionstate prior to the etching process. That is, if the initial filmthickness is within an allowable range, the film deposition apparatuscan be known to be operating normally. If it is outside the allowablerange, the apparatus can be determined as abnormal and this check resultcan be fed back.

Next, a third embodiment of the present invention will be described byreferring to FIG. 12 and FIG. 13. In controlling the etch depth, thisembodiment determines the etch depth from the initial film thickness andthe measured remaining film thickness because wafers have errors in thethickness of an organic film. The configuration shown in FIG. 12 is thesame as the configuration 11–19 of FIG. 8 and the operation of theendpoint determining circuit 230 differs from that of the endpointdetermining circuit 130 of FIG. 8. The structure of a material to beetched is shown in FIG. 13. The area of an organic film 60 to be etched(groove structure) lies where there is no mask layer 61 (e.g., nitridefilm and photoresist). The interference light observed is produced byinterference between light reflected by the surface of the organic filmand light reflected by an interconnect layer 62 (e.g., Cu).

The method of measuring an etch quantity (groove depth 65: distancebetween D and E) by measuring the interference light will be explainedwith reference to the flow chart of FIG. 14. First, a target depth ofthe material to be etched (organic film), all standard differentialpatterns associated with the organic film thickness (P_(F)j) stored inadvance in the differential waveform pattern database 16 and convergencedecision criteria (σ_(F0)) are set in the differential patterncomparator 15 (step 700). When the etching process is started, theinterference light sampling is started (step 702). The light of multiplewavelengths from the spectroscope 11 is supplied to the first digitalfilter 12, differentiator 13 and second digital filter 14 to determinesmoothed differential coefficient time series data Di,j, as in the steps604–610 of the second embodiment. These smoothed differentialcoefficient time series data Di,j are compared with the differentialpatterns P_(F)j preset in the differential pattern comparator 15 tocalculate a remaining film thickness Fi at that point in time (step715). When σ>σ_(F0), the remaining film thickness at this point in timeis not saved and the processing by the recurrent analyzer 19 excludesthe remaining film thickness data obtained at this point in time.

The remaining film thickness obtained at step 715 is stored as timeseries data Fi in the data recorder 18. By using the stored old timeseries data Fj, the recurrent analyzer 19 determines a first-orderregression line Y=Xa*t+Xb (Y: remaining film thickness, t: etch time,Xa: etch rate, Xb: initial film thickness) and, based on this regressionline, calculates the remaining film thickness F at the current time andthe initial film thickness Xb (step 716).

Next, the endpoint determining circuit 230 determines a groove depth atthe current time (=Xb−F) (65 in FIG. 13) from the remaining filmthickness F (64 in FIG. 13) and the initial film thickness Xb (63 inFIG. 13) and compares the groove depth with the target depth. If thegroove depth is equal to or more than the target depth, it is decidedthat the etch quantity of the material has reached the predeterminedvalue and the result is displayed on the display 17. When the groovedepth is less than the target depth, the procedure returns to step 704.As a final step, the sampling termination is set (step 714). In thisway, the etch depth when forming a groove can be measured by determiningthe remaining film thickness F and the initial film thickness Xb by therecurrent analysis.

Next, a fourth embodiment of the present invention will be described byreferring to FIGS. 15, 16 and 17. The configuration of this embodimentshown in FIG. 15 is identical with the configuration 11–19 of FIG. 12,and the operation of an endpoint determining circuit 330 differs fromthat of the endpoint determining circuit 230 of FIG. 12. This embodimenthas a controller 1000. One and the same etching apparatus 1 is oftenused to etch various materials with different film properties. In thatcase, the etching process is carried out by time-controlling a gassupply unit 1001, a plasma generator 1002 and a wafer bias power supply1003 by the controller 1000 according to etching conditions (e.g.,etching gas condition, plasma generating power condition and biascondition) preset in the controller 1000. When the material to be etchedhas a stacked semiconductor device structure as shown in FIG. 16,however, the etching process becomes complicated and the devicefabrication without damages is difficult to achieve with a simpletime-controlled etching process. Now the process of etching such stackeddevice structure will be described by referring to FIG. 16. Overlying apolysilicon film 70 to be etched by this etching process are a BARC 73(Back Anti-Reflection Coating) and a mask layer 71 (e.g., nitride filmand photoresist). Underlying the polysilicon film 70 is an oxide film72. The underlying oxide film 72 has a structure in which its thicknessat a transistor gate electrode portion 77 (e.g., about 2 nm) is greatlydifferent from that at a transistor device isolation groove portion 79(STI) (e.g., about 300 nm). In processing this structure, the BARC 73 isfirst etched, followed by the polysilicon film 70 being etched by thesame etching apparatus. In this etching process, if these films fail tobe etched correctly, the oxide film 72 underlying the gate electrodeportion 78 may be overetched causing damages to the device. For thisreason, the etching of the BARC 73 must be controlled so as to preventthe polysilicon film 70 from being etched as practically as possible. Itis therefore important in the BARC etching process to measure theremaining thickness of the BARC 73; when the remaining thickness 75 issmall (e.g., 20 nm), to change the etching condition to the one thatrenders the polysilicon difficult to etch; and to etch the remainingBARC 73 under this condition. Next, during the process of etching thepolysilicon film 70, it is important that the remaining thickness of thepolysilicon be measured and, when the remaining thickness 77 becomessmall (e.g., 20 nm), the etching condition be changed to the one thatrenders the underlying oxide film difficult to etch and under thiscondition the remaining polysilicon 70 be etched.

The light used to measure the remaining thickness of BARC isinterference light 96 produced by interference between light reflectedfrom the BARC surface and light reflected from a polysilicon boundarysurface. In measuring the remaining thickness of the polysilicon,interference light 95A or 95B is used which is produced by interferencebetween light reflected from the polysilicon surface and light from theunderlying oxide film boundary surface. Because the thickness 77 of theunderlying oxide film 72 at the gate electrode portion 78 differs fromthe thickness 76 at the transistor device isolation groove portion 79,the interference light intensities 95A, 95B from these portions alsodiffer, with the interference light intensity 95B from the deviceisolation portion 79 stronger than the interference light intensity 95Afrom the gate electrode portion 78. Hence, the measurement of theremaining thickness of the polysilicon is carried out based on thepolysilicon at the device separation portion 79. With the abovediscussion taken into consideration, the polysilicon etching processuses the interference light intensity 95B to perform the etching untilthe remaining polysilicon thickness is equal to the thickness 76, afterwhich the remaining polysilicon is again etched under the etchingcondition that makes the underlying oxide film difficult to etch.

The procedure for this etching process will be explained with referenceto the flow chart of FIG. 17. First, the etching conditions for thestacked films such as BARC 73 and polysilicon 70 (e.g., gas condition,discharge condition and pressure condition), target remainingthicknesses 75, 76 of the films 73, 70, and convergence decisioncriteria are set in the controller 1000 (step 800). Next, according tothe kinds of films to be processed, all standard differential patterns(P_(Z)j) for the thicknesses of the films 73, 70 stored in advance inthe differential waveform pattern database 16 and convergence decisioncriteria (σ_(Z0)) are set in the differential pattern comparator 15(step 801). At the next step the etching process and the interferencelight sampling are started (step 802). Then, according to theinstruction from the controller 1000, the standard differential patternP_(Z)j and the convergence decision criterion σ_(Z0) for the material tobe etched first (e.g., BARC) are picked up from the differentialwaveform pattern database 16 and set in the differential patterncomparator 15 (step 803). The rays of light of multiple wavelengths fromthe spectroscope 11 are supplied to the first digital filter 12 thatproduces smoothed time series data Yi,j (step 804). Further, thedifferentiator 13 and the second digital filter 14, as in the steps704–710 in the third embodiment, determine the smoothed differentialcoefficient time series data Di,j (steps 806, 808). These smootheddifferential coefficient time series data Di,j are compared with thedifferential patterns P_(F)j preset in the differential patterncomparator 15 to determine a remaining film thickness corresponding tothe minimum convergence value of σ=Σ(Di,j−P_(Z)j)². When σ≦σ_(Z0), theremaining film thickness thus obtained is taken as the remaining filmthickness value Zi at this point in time and stored in the data recorder18 (steps 810, 815). When σ>σ_(Z0), the remaining film thickness valueat this point in time is not saved and the processing by the recurrentanalyzer 19 excludes the remaining film thickness data at that point intime (step 815). By using the old stored time series data Zi, therecurrent analyzer 19 determines a first-order regression line Y=Ya*t+Xband, based on this regression line, calculates the remaining filmthickness Z at the current time (step 816).

Next, the endpoint determining circuit 330 compares the presentremaining film thickness Z with the target remaining film thickness 75issued from the controller 1000 (e.g., BARC remaining film thickness of20 nm). If the present remaining film thickness Z is equal to or lessthan the target remaining film thickness 75, it is decided that the etchquantity of the material being etched has reached the predeterminedvalue and the result is displayed on the display 17. At the same time,the etching condition is changed to another BARC etching condition underwhich the polysilicon is not easily etched, and the next etching processis performed under this condition (step 818). When the current remainingfilm thickness Z is larger than the target thickness, the procedurereturns to step 804. Under the second selected BARC etching condition(under which the polysilicon is not easily etched), the controller 1000controls the wafer bias power supply 1003 to etch the remaining portionof the film and, after a predetermined time has passed, a check is madeas to whether the etching condition needs to be switched for etching thenext film (step 819).

In the etching of the next film (e.g., polysilicon 70), the gas supplyunit 1001, the plasma generator 1002 and the wafer bias power supply1003 are controlled to conform to the preset etching condition. At thesame time, the standard differential patterns (P_(Z)j, σ_(Z0)) of theremaining thickness 76 of the film to be etched (e.g., polysilicon) areset in the differential pattern comparator 15 (step 803) and the steps804 to 818 are again executed. The endpoint determining circuit 330detects when the remaining polysilicon thickness Z is equal to thetarget thickness 76 (e.g., remaining polysilicon thickness of 20 nm)(step 818) and displays the detection result on the display 17. At thesame time, the etching condition is changed to another polysiliconetching condition under which the underlying oxide film is not easilyetched) (step 818). This second etching (under the polysilicon etchingcondition that renders the underlying oxide film difficult to etch) istime-controlled to etch the remaining portion of the polysilicon and,after a predetermined time has passed, a check is made as to whether theetching condition needs to be switched for etching the next film (step819). When no further etching is to be performed, the etching processtermination and the sampling termination are set (step 814).

As described above, with the method of the second embodiment describedabove, the remaining film thickness can be estimated even when thedifferential pattern database associated with the remaining thickness ofthe material being etched is not sufficient. Further, in generating thedifferential pattern database for the remaining film thickness, thesample wafer does not need to be etched completely and the number ofsample wafers can be reduced.

Further, because the etch rate can be determined by the recurrentanalysis of this invention, the etch rate can be controlled for eachwafer during the mass production, thus preventing possible failures ofproduct wafers.

Further, because the initial thickness of the material being etched canbe calculated by the recurrent analysis of this invention, the feedbackof the calculated result can control the overall process involving thefilm deposition apparatus and the etching apparatus during the massproduction.

According to the method of the third embodiment, even when there arevariations in the initial thickness of the material to be etched, it ispossible to calculate the current etch depth with high precision andcorrectly determine when the current etch depth is equal to the targetetch depth.

Further, with the method of the fourth embodiment, in the process ofetching a stacked film structure the overetching of the underlying filmcan be minimized by measuring the remaining thickness of each film and,at a predetermined remaining film thickness, changes the etchingcondition to another condition.

Further, in the etching of a polysilicon of a gate electrode, measuringthe remaining polysilicon thickness by the interference light from thedevice isolation portion can precisely determine when the remainingpolysilicon thickness reaches the target thickness, thus preventing theunderlying oxide film of the gate electrode portion from beingoveretched and minimizing the number of faulty wafer products.

In these embodiments described above, light of multiple wavelengths isradiated from the spectroscope having a light source to produceinterference light by interference between rays of light reflected fromthe material being processed. The interference light is used to measurethe thickness of the material being etched. It is also possible to use aspectroscope without a light source and use, as a light source, light ofmultiple wavelengths emitted from a plasma.

The present invention is further characterized by the following:

(1) An etching method which stores data measured while etching a sample,estimates an etch depth in the etching process by using the stored data,and stops the etching process at a predetermined etch depth.

(2) In etching an object material having no underlying film, an etchingmethod which measures an amount of etch in a mask formed over the objectmaterial to be etched and a step height or distance from the uppersurface of the mask to the bottom surface of the object material andthereby controls the etch depth of the object material.

(3) An etching method which estimates a remaining film thickness of aportion of the sample being etched by using the interference light dataobtained during the process of sample etching and stops the etching at apredetermined remaining film thickness.

(4) In the damascene etching process, an etching method which estimatesan initial film thickness and determines a groove thickness by usinginterference light data obtained during the etching process and stopsthe etching at a predetermined groove depth.

(5) In etching a gate layer formed over an underlying film with adifferent in-plane thickness, an etching method which measuresinterference light from a thick portion of the underlying film tomeasure the remaining thickness of the gate layer formed over the thickportion of the underlying film, thereby controlling the thickness of thegate layer.

(6) In etching an object material consisting of a plurality of stackedfilms, an etching method which measures interference light from theobject material and selects one of digital filters for each film toprocess interference light data, thereby controlling the thickness ofeach film.

(7) In BARC etching, an etching method which uses interference lightfrom the material being etched to control the thickness of the BARC andthereby prevent a possible overetch of the underlying film.

(8) In the manufacture of semiconductor devices having STI portions, asemiconductor device manufacturing method which, when etching apolysilicon that forms a part of the semiconductor devices, controls thepolysilicon etching by the remaining thickness of the polysilicon formedover the STI portion.

1. A method of processing a semiconductor wafer comprising: a step ofsetting a standard pattern consisting of a time differential value of aninterference light for each of multiple wavelengths from a first film tobe processed overlying a second film that corresponds to a predeterminedfilm process quantity of the first film; a step of etching a samestructure on the semiconductor wafer as the first film overlying thesecond film, measuring an intensity of an interference light for each ofthe multiple wavelengths from the first film and determining an actualpattern consisting of a time differential value of the measuredinterference light intensity for each of the multiple wavelengths; astep of determining a remaining thickness of the first film based on thestandard pattern consisting of time differential values and the actualpattern consisting of time differential values; and a step of etchingthe first film remaining after a determination of the thickness of thefirst film reaching a predetermined value with a changed etchingcondition during a predetermined time period.
 2. A method of processinga semiconductor wafer according to claim 1, wherein: the changed etchingcondition is a condition under which a material of the second film isnot easily etched.
 3. A method of processing a semiconductor waferaccording to claim 2, further comprising: a step of exterminating theremaining thickness of the first film based on a recurrent analysisusing an old etch quantity of the first film obtained in the step ofdetermining the remained thickness of the first film.
 4. A method ofprocessing a semiconductor wafer according to claim 1, furthercomprising: a step of exterminating the remaining thickness of the firstfilm based on a recurrent analysis using an old etch quantity of thefirst film obtained in the step of determining the remained thickness ofthe first film.
 5. A method of processing a semiconductor waferaccording to claim 1, wherein the second film is the film with adifferent in-plane thickness, and said standard pattern and actualpattern consists of the interference lights from a thick portion of thesecond film.
 6. A method of processing a semiconductor wafer accordingto claim 5, wherein: the changed etching condition is a condition underwhich a material of the second film is not easily etched.
 7. A method ofprocessing a semiconductor wafer according to claim 6, furthercomprising: a step of exterminating the remaining thickness of the firstfilm based on a recurrent analysis using an old etch quantity of thefirst film obtained in the step of determining the remained thickness ofthe first film.
 8. A method of processing a semiconductor waferaccording to claim 5, further comprising: a step of exterminating theremaining thickness of the first film based on a recurrent analysisusing an old etch quantity of the first film obtained in the step ofdetermining the remained thickness of the first film.